1. Field
This invention relates to a method for optical characterisation. The method according to the invention is more particularly suited to the characterisation of porous materials used in microelectronic integrated circuits.
2. Description of Related Art
The interconnect structures for microelectronic integrated circuits, for example transistors, are normally produced from metallic lines, typically made out of Aluminium or Copper. These metallic lines are separated by a dielectric material which serves as insulation as much in a lateral manner for the lines as in a vertical manner for the levels of metal. The problem involves overcoming deterioration in the electrical characteristics of the dielectric materials located between the metallic interconnect lines when the size of said lines is reduced. Indeed, improving transistor properties is connected to the reduction in their sizes, which leads to a reduction in the sizes of the lines and spacing between these lines. Unfortunately, moving these lines closer together causes interline parasitic capacitances to appear.
In a known manner, one means for maintaining performance levels involves replacing the dielectric material of the interlines with a low dielectric constant material, enabling the capacitive coupling between the adjacent lines to be reduced. One solution for lowering the dielectric constant consists in introducing porosity into the dielectric material. A major difficulty however resides in the notion of conserving the porous properties of the material during its integration into microelectronic circuits.
Therefore, the porous properties of these low dielectric constant materials must be capable of being controlled.
A first known solution enabling the porosity and size of the pores to be characterised consists in using an adsorption technique coupled with a mass measurement. These weight measurements nevertheless require dense samples in order to observe a significant variation in mass during the adsorption process.
Another known solution enabling the porosity and size of pores to be characterised for materials placed in a thin layer consists in using an adsorption technique coupled this time with an ellipsometric measurement. Such a process is described in particular in the patent application WO 00/12999. Nevertheless, such a solution presents some disadvantages.
Indeed, when integrating the porous materials into microelectronic circuits, a problem arises involving the capacity to measure the properties of said materials during the integration process, i.e. when they are no longer in the form of thin films, but in the form of patterns originating from standard lithography and etching processes, as the measurements taken by porosimetric ellipsometry are suitable for measuring thin porous films. Such a solution therefore does not enable the porous material to be characterised close to its final use, i.e. when it is in the form of porous patterns originating from lithography and etching processes. This pattern characterisation is fundamental insofar as the properties of the porous materials can evolve during the lithography and etching processes.